A gcc built against glibc-2.30 (and some earlier releases) will fixinclude <bits/statx.h> and store a modified copy in its installation.  This copy differs from glibc's as follows:

@@ -1,3 +1,12 @@
+/*  DO NOT EDIT THIS FILE.
+
+    It has been auto-edited by fixincludes from:
+
+       "/tmp/cross-x86_64/usr/include/bits/statx.h"
+
+    This had to be done to correct non-standard usages in the
+    original, manufacturer supplied header file.  */
+
 /* statx-related definitions and declarations.  Linux version.
    Copyright (C) 2018-2019 Free Software Foundation, Inc.
    This file is part of the GNU C Library.
@@ -26,7 +35,7 @@
 
 /* Use "" to work around incorrect macro expansion of the
    __has_include argument (GCC PR 80005).  */
-#if __glibc_has_include ("linux/stat.h")
+#if __glibc_has_include ("__linux__/stat.h")
 # include "linux/stat.h"
 # ifdef STATX_TYPE
 #  define __statx_timestamp_defined 1

The incorrect replacement of linux with __linux__ is tracked as PR91085, but that is not the issue reported here.

Now assume this compiler installation is used with glibc-2.31 (for instance, due to an OS upgrade), and try to compile the following:

> cat test.c
#define _GNU_SOURCE
#include <sys/stat.h>
int main (void) { return 0; }
> /tmp/cross-x86_64/bin/x86_64-unknown-linux-gcc -Wall -O2 -c test.c
In file included from /tmp/cross-x86_64/usr/include/sys/stat.h:446,
                 from test.c:2:
/tmp/cross-x86_64/lib/gcc/x86_64-unknown-linux/8.4.0/include-fixed/bits/statx.h:38:25: error: missing binary operator before token "("
 #if __glibc_has_include ("__linux__/stat.h")

This error occurs because the fixincluded <bits/statx.h> refers to the __glibc_had_include () macro which was removed in glibc-2.31.

If this gcc is now used to bootstrap another on on the glibc-2.31 system, lots of configure tests fail, and the build errors out in libiberty:

  gcc -c -DHAVE_CONFIG_H -g  -I. -I/tmp/gcc-9.3.0-RC-20200305/libiberty/../include  -W -Wall -Wwrite-strings -Wc++-compat -Wstrict-prototypes -Wshadow=local -pedantic  -D_GNU_SOURCE -fpic /tmp/gcc-9.3.0-RC-20200305/libiberty/regex.c -o pic/regex.o; \
else true; fi
In file included from /tmp/gcc-9.3.0-RC-20200305/libiberty/regex.c:38:
./config.h:568:15: error: two or more data types in declaration specifiers
 #define pid_t int
               ^~~
./config.h:571:17: error: two or more data types in declaration specifiers
 #define ssize_t int
                 ^~~
make[3]: *** [Makefile:1167: regex.o] Error 1
make[3]: Leaving directory '/tmp/objdir/libiberty'
make[2]: *** [Makefile:11197: all-stage1-libiberty] Error 2
make[2]: Leaving directory '/tmp/objdir'
make[1]: *** [Makefile:21422: stage1-bubble] Error 2
make[1]: Leaving directory '/tmp/objdir'
make: *** [Makefile:993: all] Error 2

This error is the result of configure tests that failed due to direct or indirect includes of <sys/stat.h>.  This is what I noticed after an upgrade from Fedora 30 to Fedora 31: my existing gcc:s could no longer bootstrap new ones.

While one could argue that glibc-2.31 was in error to remove the __glibc_has_include () macro, gcc is also in error for creating that unnecessary dependency in the first place: it should not have fixincluded <bits/statx.h>.

The error is easily reproduced by building a cross with glibc-2.30 and then updating it to glibc-2.31 (which is why some of the samples above mention a cross rather than a native compiler).